To maximize social welfare under transportation sustainability, bi-level budget allocation models are proposed, where the upper level maximizes the social welfare of trip makers by allocating budget to subsidize bus fare discount and bus frequency and to acquire green land for accommodating excess footprint. Two models are developed: the single generation (SG) model and the across generation (AG) model. The SG model assumes these decisions are made under the consideration of the contemporary generation alone, while the AG model compromises these decisions with the consequent generation. A case study on an exemplified network is conducted. Results show that the measures of bus fare discount and bus frequency increase can attract remarkable bus patronage and achieve almost the same social welfare. The optimal decision of the contemporary generation compromises the total utility of the contemporary generation with that of the next generation by leaving partial budget to the next generation. In contrast to traditional transit network design model, the proposed model further considers the trade-off and decisions across generations.